With bistatic radar systems, it is generally not possible to detect targets that are outside the area formed by the intersection of the transmit and the receive beams. Consideration must therefore be given to how radiating radar energy might most effectively utilize to search an area. One such approach is described by Merrill I. Skolnik in the "Radar Handbook" (McGraw Hill 1970), in which multiple receive beams are simultaneously formed so they cover the entire area illuminated by the transmit beam.
A shortcoming of this scheme, however, is that when the transmitter and receiver are separated by large distance or the surveillance area is extensive, many receive beams are needed, which increases the scale of the equipment to the point it becomes impractical.
A prior art approach to resolve this problem is known as "pulse chasing," as described for example by E. Hanle in a paper entitled "Survey of Bistatic and Multistatic Radar" (IEEE Proceedings, PP 592-594, Vol. 133, Part F, No. 7, 1986). In pulse chasing, the receiver is synchronized to the transmitter so that the receive beam scans rapidly tracking the propagation of radiated energy in the transmit beam.
Referring now to FIG. 5, the prior art configuration of the pulse chasing approach is illustrated.
The principal components of the configuration are a transmitter 1, a transmit antenna 2, a transmit beam controller 3 that produces the transmit beam scanning signal, containing data indicative of the time of transmission, and data indicative of the directional angle, a transmission path 11 for transmitting said transmit beam scanning signal, a receive antenna 5 comprising antenna elements for receiving signals reflected from a target 4, a receive beam controller 6 that produces the beam scanning signal required to scan the receive beam at high speed, in accordance with the transmitted scanning signal and an equation (1) to be set forth later, a digital beamformer 7 responsive to the receive beam scanning signal, for forming the desired receive beam by digitally synthesizing the signals received by the antenna elements of receive antenna, a signal processor 8 for pulse compression and Doppler processing, a data processor 9 that extracts the target information from the receive signal, detects the position (calculates the position coordinates), and performs tracking processing, and a display device 10.
Transmitter 1 located at the transmit station T provides an rf signal and sends it to transmit antenna 2, which causes the signal to be radiated into space in the desired direction. The beam formed at transmit antenna 2 successively scans the required surveillance region based on a beam scanning signal generated by beam controller 3. If the transmit beam illuminates target 4, then reflected energy is sent back to receive antenna 5 at the receive station R. In synchronization with the propagation of the transmit signal, the receive beam rapidly scans across the area illuminated by the transmit beam, as shown in FIG. 6. The scan rate of the receive beam is given by ##EQU1## where -.delta..phi..sub.R /.delta. t is the scanning angular velocity of the receive beam, c is the speed of light, b is the distance separating transmit station T and receive station R, .phi..sub.T is the angle formed by the baseline TR connecting transmit and receive stations T and R, and the transmit beam, and similarly, .phi..sub.R is the angle formed by the baseline TR and the receive beam. It will be apparent from equation (1) that the scanning rate of the receive beam is dependent on the directional angle .phi..sub.T of the transmit beam. The scan process is coordinated by sending the transmit beam scanning signal, containing the data indicative of the time of transmission, and the data indicative of the directional angle .phi..sub.T, produced by transmit beam controller 3 at the transmit station to the receive beam controller 6 at the receive station R, where the receive beam scanning signal is produced in accordance with the transmit beam scanning signal and the equation (1). Based on this scanning signal, digital beamformer 7 forms the desired receive beam by digitally integrating the signals received by the constituent elements of receive antenna 5.
The principles of a digital beamformer are generally known by those acquainted with the art, and is therefore omitted here. A good summary treatment of the subject can be found in the article "Digital Beamforming Antennas, An Introduction" (Microwave Journal, PP 107-124, January 1987). by H. Steyskal.
Once it is received as described above, the receive beam scanning signal undergoes necessary processing by signal processor 8 including pulse compression and ranging processing, as well as Doppler processing to suppress clutter, which includes moving target indication, and pulse-Doppler processing. After that, data processor 9 performs target signal extraction, position detection, and tracking processing. For the position detection, receive beam directional angle information from the receive beam controller 6 is generally employed in addition to the ranging processing results. The results of the processing are then shown on display device 10.
Since only one receive beam is employed in this prior art configuration, a fairly broad beamwidth must be employed to assure that none of the area illuminated by the transmit beam is missed. We will now consider this situation in more detail, making reference to FIGS. 7 and 8.
The first constraint affecting the width of the receive beam is the width of the transmit beam. As shown in FIG. 7, the set of target positions with the signals reflected therefrom being received at the receive station R at one time is on an ellipse whose foci are the transmit and the receive stations. If the transmit beam cuts across this ellipse from points A to B, then the width of the receive beam must be broad enough to cover any prospective angle formed by these two points. Taking this constraint into account, the minimum width .theta..sub.R1 of the receive beam is given by ##EQU2## where .theta..sub.T is the transmit beam width, .phi..sub.T is the angle formed by the baseline connecting transmit and receive stations and the transmit beam, and .phi..sub.R is the angle formed by the baseline and the receive beam. It is clear from equation (2) that the width of the receive beam must increase in proportion to that of the transmit beam.
The second constraint on the width of the receive beam is the width of the transmit pulse. FIG. 8A shows a schematic diagram of the relationship between the transmit and receive beams, and FIG. 8B shows a timing chart of the receive signals. Assume that signals P.sub.A emitted from transmitter T and reflected from target A begin to reach receive station R at time t.sub.A. If transmission begins at time t=0, then adopting the symbol employed in FIG. 8A, t.sub.A is derived as follows: ##EQU3## where c is the speed of light. TA is the distance from the transmit station T to the target A, and RA is the distance from the target A to the receive station R.
Now if we let .tau. represent the transmit pulse width (or the transmit frame-time in the case of CW radar), then signal P.sub.A reflected from target A is received in the time-frame t.sub.A to (t.sub.A +.tau.). The receive beam must therefore illuminate target A during this time frame. Next, we consider target B detected by the same transmit beam at the position EQU TB+RB=TA+RA+c.tau. (4)
Referring again to FIG. 8B, since signal P.sub.B reflected from target B is received during the time-frame t.sub.B to (t.sub.B +.tau.), it is apparent the receive beam must begin illuminating target B at time t.sub.B. Now, since we can derive ##EQU4## from equation (4), it is apparent that the receive beam must illuminate both targets A and B simultaneously at time t.sub.A +.tau. (=t.sub.B).
Expressing the relationship more formally, the required width of the receive beam .theta..sub.R2 is given by ##EQU5## Equation (6) reveals that the longer the transmit beam's pulse width .tau., the broader the width of the receive beam must be made.
We can represent the width of a receive beam .theta..sub.R that would satisfy both constraints discussed above as follows: EQU .theta..sub.R =max (.theta..sub.R1,.theta..sub.R2) (7)
From this it is apparent that when the transmit beam has a broad width, or when the transmit pulse width or transmit time-frame is long, then the beamwidth of the receive beam must also be made correspondingly broad.
While for convenience we have assumed that transmit antenna 2 for electronic scanning is a phased array antenna, it will be apparent to those familiar with the art that the antenna could just as easily be implemented with mechanical beam scanning, in which case transmit beam controller 3 would be replaced by a beam directional angle detector. Similarly, receive antenna 5 has been described as a digital beamforming antenna that works in conjunction with digital beamformer 7, but it could just as readily be implemented as a phased array antenna featuring electronic scanning through phase control over each antenna element.
From this description of the prior art bistatic radar configuration, it is clear that the width of the receive beam must be broad if the width of the transmit beam is wide, or if the transmit pulse (in the case of pulse radar) or frame-time (in the case of CW radar) is long. Various problems are associated with a broad receive beam: the detection range is reduced, the target resolution and position accuracy are diminished, and the radar is more susceptible to clutter and other kinds of interference.